Problem: On the first 3 exams of her history class, Ashley got an average score of 78. What does she need on the next exam to have an overall average of 82?
Let her score on the next exam be $x$ The sum of all of her scores is then $3 \cdot 78 + x$ The same sum must also be equal to $4 \cdot 82$ Solve: $x = 4 \cdot 82 - 3 \cdot 78 = 94$.